The following art appears relevant to the present disclosure:
U.S. Patent Application PublicationsPublicationKind NumberCodeIssue DatePatenteeUS2012/0250909A1Oct. 4, 2012Grosh
Micromachined ultrasonic transducers (MUTs) have been subject to extensive research for the last two decades. Piezoelectric micromachined ultrasonic transducers (pMUTs) are MUTs that use a piezoelectric layer for electro-mechanical transduction. A typical pMUT is a multilayer membrane structure that is excited into flexural vibration using piezoelectric actuation. The membrane structure is often formed by etching through a silicon wafer to remove the material beneath the membrane, thereby allowing it to vibrate. This etch forms a hollow tube beneath the backside of the membrane. Sound is emitted from the tube when the membrane vibrates, and the tube may be designed as an acoustic resonator to improve acoustic performance of the pMUT. These devices typically operate at the membrane's flexural resonance frequency, which is defined by selecting the correct materials, membrane size, thickness and/or in-plane stress. For multi-pMUT applications, good matching of the resonance frequencies of the individual pMUTs is required for proper operation. For this reason, it is important for pMUTs to be manufactured with closely-matched resonance frequencies. One important parameter causing frequency variation is the residual stress present in the layers composing the pMUT membrane, in particular in the piezoelectric layer. A typical pMUT structure consists of a membrane that is attached to the substrate at its boundary, a condition that is described as a clamped boundary condition. The resonant frequency of a membrane with a clamped boundary condition is very sensitive to in-plane residual stress. Several designs have been suggested to reduce stress sensitivity, such as released cantilevers and flexurally-suspended membranes, but they all show poor acoustic performance and or poor manufacturability (e.g. inefficient resonance modes, cracks created by stress concentrations, poor micro-fabrication yield)
In US 2012/0250909 Grosh describes an acoustic transducer wherein a membrane transducer is released from the substrate by separating the membrane into several identical tapered cantilevers, reducing the effect of stress on the mechanical behavior of the membrane. Grosh's approach works well for a non-resonant device, such as a conventional piezoelectric microphone. However, for a pMUT operated at resonance, small differences caused by fabrication variations can cause the cantilevers to have slightly different resonance frequencies, resulting in considerable negative impact on the acoustic performance of the device when operated at resonance. Specifically, when excited at a single frequency, mismatched cantilevers will oscillate with significantly different phase and amplitude, creating phase and amplitude errors in the ultrasound signal.
Accordingly, what is needed is a pMUT design with good acoustic performance that resonates at a single stable resonance mode, and with low sensitivity to stress. This invention describes such design.